POWER GENERATION AND TRANSMISSION 55 Size of wire, B. & S. gage Maximum allowable current-amperes Rubber insulation Other insulation 6 50 70 4 70 90 2 90 125 0 125 200 0000 225 325 Circular mils 500,000 400 600 1,000,000! 650 1,000 1 Used mainly for underground work. drop of 5 or 10 per cent is allowed: for secondary mains and service wires, 2 to 5 per cent. The diversity factor (sum of maximum demands of the parts of a system, divided by the maximum demand of the whole system), enters into the matter. If I = current in amperes, v = drop per conductor in volts, I — length of one conductor in feet, the actual cross-section of copper necessary, in circular mils, is A = Idl -5- v, where k = 11 for direct current. For alternating current, k varies with the power factor, etc., and may be as high as 17 at 0,80 power factor. Wiring Specifications.—Interior wiring may be: (a) Open.—On cleats or insulators: rubber insulated if in damp places. (b) Moulding.—For under 300 volts: forbidden in concealed or damp places: no joints or taps in the moulding: rubber insulated: wood moulding forbidden in elevator shafts: metal moulding limited to 1,320 watts and must be completely connected and grounded. (c) Concealed Knob and Tube.—Often forbidden, but cheap. Rubber insulation: wires 5 in. apart, 1 in. from adjacent surface, supported every 53 in. (d) Rigid Conduit.—Most expensive type. Sizes J^ to 4 in. Grounded. Rubber insulation. Lined or unlined conduit may be obtained, the "lining" consisting of a layer of insulation. (e) Flexible Conduit.—Interlocking metallic strips. Sizes %Q to 2 in. Specifications otherwise as for rigid conduit. (/) Armored Cable.—Rubber insulated and covered with interlocking metallic strips. Direct current is generally distributed by two-wire or three-wire constant "potential" systems. The latter (with all three wires of the same size) saves five-eighths the copper necessary for the former. The gross saving is less. Alternating current is distributed as: Two-phase, Three-wire.—Each single-phase current has its own outgoing wire. The return wire serves both. Voltage between two outside wires is 41 per cent more than that between outside and return wire. Current in return wire 41 per cent greater than in either outside wire. An unbalanced system. Two-phase, Four-wire.—Two complete single-phase circuits. The neutrals of the two circuits may be connected, in which case the voltage across circuits is 71 per cent of that between the wires of a phase. Extensively used. In the five-wire system, the common neutral is extended as a fifth wire.